25. Theories and patterns of eruption.

THEORIES AND PATTERNS OF ERUPTION

ENTIRELY WRONG THEORIES explaining the causes of dental eruption:

• Theory of muscular action on chewing act on the alveolar bone
• Alveolar bone resorption theory
• Theory of pulp pressure and growth

POSSIBLE THEORIES

ROOT THEORY

• Tooth erupts due to an increase in root length. In its growth, the root pushes the crown towards the mucous membrane
• Evidence disproving the theory:
• Tooth erupts without its root being formed
• During eruption, tooth travels a distance greater than its root length
• In experimental conditions, tooth also erupts when root is removed

THEORY OF DENTIN GROWTH AND PULP CONSTRICTION

• With apical growth of root, the pulp is compressed apically and creates occlusal forces
• Evidence disproving the theory:
• At the time of the eruption, Apical opening is wide and pulp is remodelled parallel to the dentin and no pressure is obtained

THEORY OF BONE GROWTH

• Reason for eruption is due to the accumulation of bone beams in the bottom of the alveoli
• Evidence disproving the Theory:
• Evidence that bone apposition is a consequence of occlusal displacement of the tooth, not the cause of eruption

Contemporary Opinion Theory

• All factors that influence the tooth eruption act simultaneously
• Erupting tooth moves from area of increased pressure to an area of decreased pressure

PATTERNS OF TOOTH ERUPTION

• Teeth are erupted in pairs. There is symmetry in the eruption of the left and right sides
• Lower teeth are erupted before the upper teeth, except for the premolars which first erupted in the upper jaw
• Teeth erupt in a certain sequence
• Primary Teeth Sequence
• Lower central, upper central, lower lateral, upper lateral, upper 1st molar, lower 1st molar, lower canine, upper canine, lower 2nd molar, upper 2nd molar
• Permanent Teeth Sequence
• Lower 1st molar, upper 1st molar, lower central, upper central, lower lateral, upper lateral, upper 1st premolar, lower 1st premolar, lower canine, upper canine, upper 2nd premolar, lower 2nd premolar, lower 2nd molar, upper 2nd molar
• Tooth eruption starts with primary 1st lower central incisor for about 6 months
• General rule for eruption is 6/4= every 6 months, 4 teeth are erupted
• Primary dentition is formed by eruption of second molar at around 2-2.5yrs
• Mixed dentition from 6th year when erupted, either first permanent incisor or first permanent molar
• Mixed dentition lasts until replacement of second primary molar with second molar. Permanent dentition is ready at 12 years
• Primary teeth replaced earlier and permanent teeth erupt earlier in girls